This invention relates to a method of recovering phosphorus from phosphorus-containing sludge. In particular, it relates to the treatment of this sludge with a phosphate salt which, on further processing, causes the phosphorus particles to coalesce into a separate phosphorus phase.
Sludge from manufacturing phosphorus can contain small amounts of phosphorus and dirt particles dispersed in an aqueous phase. The phosphorus should be removed from the sludge for both economic and environmental reasons. The phosphorus cannot be recovered from the sludge by filtration because most of the filtered phosphorus stays dispersed in water. Phosphorus can be recovered from this sludge by heating the sludge in the absence of air to evaporate the water, but that is uneconomic. Various substances can be added to the sludge to cause the phosphorus particles to coalesce but, until now, this has not been very effective.
I have discovered that the addition of certain phosphate salts to a heated aqueous phosphorus-containing sludge followed by filtration causes the dispersed phosphorus particles to coalesce and form a continuous phosphorus layer. The phosphorus can then be easily separated and recovered as a valuable product.
This is a very cost-effective process for recovering phosphorus as a valuable product from sludge and for converting the sludge into an inert non-hazardous waste for disposal.
This invention is applicable to aqueous sludges containing dispersed phosphorus particles as a discontinuous phase. The amount of phosphorus in the sludge should be at least about 1 wt % (based on sludge weight) because it is usually not economical to treat sludges containing less phosphorus by the method of this invention. Sludges that contain more than about 15 wt % phosphorus and have a continuous phosphorus phase can usually be treated more economically by other methods. Preferably, the sludge contains about 5 to about 10 wt % phosphorus. The phosphorus is present as water-dispersed particles of P4 that are too small to readily settle out. Particles of xe2x80x9cdirtxe2x80x9d and other charged impurities may keep the phosphorus particles in suspension and prevent them from agglomerating. The xe2x80x9cdirtxe2x80x9d is a mixture of a variety of substances, such as coke, sand, phosphate rock, etc.
The aqueous sludge is placed in a tank and a phosphate salt that has the general formula 
is added to it, where R is an alkali metal or ammonium and n is 0 to 30. Preferably, n is 0 to 12 and R is sodium or potassium as many of those salts are commercially available; sodium salts are most preferred as they are less expensive. Many of the phosphate salts are commercially available and those that aren""t can be made using processes similar to the those used to prepare the commercially available phosphate salts. The preferred phosphate salts are sodium tripolyphosphate (STPP), tetrasodium pyrophosphate (TSPP), and sodium hexametaphosphate (SHMP), which have the respective formulas: 
where m is 6 to 30. These phosphate salts are preferred as they have been found to work well and are commercially available.
The amount of phosphate salt added to the sludge should be about 1 to about 20 wt %, based on the weight of the phosphorus in the sludge, as less is not very effective and more is usually unnecessary and without additional benefit; preferably, about 2 to about 10 wt % of the phosphate salt is added.
Before or after the addition of the phosphate salt, the sludge is heated above the melting point of phosphorus, 44.1xc2x0 C.; preferably, it is heated to about 50 to about 70xc2x0 C. It is preferable to stir for about 5 to about 60 minutes while heating. The sludge is then filtered, preferably under a pressure of about 10 to about 100 psi, or it can be centrifuged. Water and the liquid phosphorus in the aqueous phase pass through as the filtrate and the dirt forms a filter cake. Filtered or centrifuged phosphorus particles agglomerate or coalesce in the aqueous phase and sink to the bottom of the tank, forming a liquid phosphorus phase. This phosphorus phase can be easily separated from the aqueous phase by gravity draining, pumping it out, decantation of the water, or other techniques. The process can be run continuously or in batches.
The following examples further illustrate this invention: